Polymers based on vinylidene fluoride CF2═CH2 (VDF), such as, for example, PVDF (polyvinylidene fluoride) homopolymers and copolymer, are known to offer excellent mechanical stability properties, very great chemical inertness and good resistance to ageing. These qualities are useful in varied fields of application.
Additives are often added to fluoropolymers to improve their properties. For example: flame retardancy (U.S. Pat. No. 7,642,313), and whiteness after heat processing (adding sodium acetate as described in U.S. Pat. Nos. 7,045,584 and 7,012,122). White pigments, such a TiO2 and ZnO have been added to improve the fluoropolymer whiteness, the whiteness can be diminished during heat processing.
U.S. Pat. No. 7,192,646 describes the use of 5 to 15 percent of acid acceptors in a fluoroelastomer used in fuel hose. The acid acceptors include magnesium oxide, calcium hydroxide, litharge, dibasic lead phosphate, calcium oxide, and zinc oxide.
Irradiation of fluoropolymers may be done for several reasons, such grafting of functional groups (such as the grafting of maleic anhydride onto a fluoropolymer as described in U.S. Pat. No. 7,241,817), to create branching and enhance properties (U.S. Pat. No. 7,514,480), and for sterilization of articles containing fluoropolymers (U.S. Pat. No. 5,516,564).
Many high-purity operations require clean, pure processing environments and containers. Polymers used in these applications must be extremely chemical resistant and capable of being easily sterilized. The high-purity polymers find use in applications including bags or other containers for high purity fluids, biological and biomedical media, as well as high purity chemicals and reagents. Fluoropolymer articles cannot always be easily sterilized in a steam autoclave, as they will melt together and become useless.
Fluoropolymers are known for their stability, and fluoride ion is very difficult to leach or extract from a fluoropolymer.
Unfortunately, fluoropolymers exposed to irradiation and other high energy radiation, can undergo scission of some polymer bonds, or can create carbon-carbon double bonds (which can cause decoloration), with the release of small amounts of fluoride ions and small fluoride-containing molecules. Other fluorinated compounds used in the polymerization of a fluoropolymer, residual monomer, and oligomers can also release fluoride ions and small fluoride-containing molecules. In addition to the fluorine ion, other leachable fluorine-containing small molecules include, but are not limited to: HF, fluorine-containing monomers and oligomers, and fluorinated surfactants. The fluoride ion is extremely reactive. While the concentration of extractable fluoride ion is low, there is a desire to reduce the level even further—especially in applications in which the fluoropolymer comes in contact with the human body, or in contact with fluids meant for contact with living organisms. In these cases leachable or extractable fluorine compounds need to be minimized to levels that are tolerable by the human body.
Surprisingly, it has now been found that the addition of small levels of some metal salts or oxides to fluoropolymers, substantially reduce the migration of the fluoride ion into the material contained in these vessels, especially after the fluoropolymer undergoes treatment by irradiation.
An added advantage of the invention is that the reduction of the fluoride ion concentration tends to reduce discoloration of the fluoropolymer, leading to a whiter article.